Masdyman - your impressions after feeding at that rate?

I've been trying to use EC to monitor fertiliser use, shooting for ~400 uS for cattleyas and Paphs, ~600 for coelogyne, lycaste, oncidium, masdevallia and ~800 for phals and cymbidiums. I've been making up really weak (~200uS) for Disa. This took quite a bit of mucking about, as you can imagine.

I recently received some RO MSU fertiliser from FirstRays, and just now mixed up a batch - 1 tsp in 5l of water, which should be about 125ppm N if my maths worked out OK. I then put my TDS meter in there, and was surprised to find the EC was about 1269uS/cm - pretty high!

I'm somewhat worried I'll end up harming my plants as I've been using much weaker solutions than this in the past (and being a newbie, I fret constantly...!). Plants are not in S/H; they're mounted, in bark, or in sphag depending on the plant.

Advice? :)

I doubt that the much weaker solution will have harmed your plants. Just switch over to the 125ppm equivalent and they should be fine, and probably will do even better for you. 125ppm N is still a very weak dose of fert if you compare it to what non orchids get! For the salt intolerant orchids (masdies and such) a good benchmark figure is about 70-80ppm N.

If you are worried about burning the roots of your plant then the important metric is not ppm of nitrogen but the ionic strength of the fertlizer solution. I could make a fertilizer solution with only 125 ppmN that would be guaranteed to kill orchids - just add 1% table salt.

Instead you should be measuring electrical conductivity (TDS meter) as a measure of the ionic strength of your fertilizer solution.

Worrying about ppm N is for large scale hydroponic operations where they are performing chemical analysis of their of their solution and adding make up chemicals.

I use Dyna-Gro Orchid Pro at 1:1500 in RO water. This gives me a TDS reading of about 200 ppm.
Jim’s orchids supplies now offers dyna gro liquid fertilizers and supplements. Including, liquid grow and bloom, orchid pro, pure neem oil, kln rooting liquid, pro tekt and rooting gel.

I like the Dyna-Gro liquid over the "MSU" powder formulations because I question how well blended the "MSU" powders are.

Wow, that is quite high! 1270 microSiemen/cm X 0.67 = 813 ppm TDS.
Conductivity convertor, Siemens into TDS PPM and MHO